This invention relates to data acquisition apparatus for use in a wellbore and a method of acquiring signals down a wellbore.
Data acquisition apparatus, including sensors, is often provided in a wellbore to monitor the movement of fluid within a fluid producing reservoir or movement of fluid within the wellbore itself. In a wellbore with casing and tubing in place, sensors are typically placed behind the tubing, so as to be in an annulus between tubing and casing, and the sensors are connected to the surface by a cable positioned in the annulus. The cable provides both power and telemetry to the sensors. It has been attempted to deploy the sensors behind casing, connecting the sensors to the surface by a cable which passes through the cement holding the casing in contact with the outer wall of the wellbore. However problems have arisen with such techniques due to the difficulties of protecting the cable during installation, and ensuring that the quality of the cement is not affected by the presence of the cable, as otherwise zonal isolation cannot be maintained.
It has been proposed to avoid such problems by providing a wireless telemetry link from downhole to surface. However problems also arise with this approach in relation to signal attenuation and transmission range.
According to one aspect of the present invention, there is provided a data acquisition apparatus for use in a wellbore, comprising a receiving means attached to an elongate electrical connector for conveying signals from downhole to surface, and a plurality of sensing devices adapted to be positioned downhole in spaced apart positions remote from the receiving means, so as to form an array of sensing devices.
Typically the electrical elongate connector is a cable attached to a surface processing unit which can both receive and transmit signals along the connector.
Preferably the receiving means includes a transmitter so as to allow two-way travel of signals within the wellbore. Thus both uphole and downhole transmission of signals is enabled, allowing acquired data to be forwarded uphole and command signals from the surface to be sent downhole to the sensing devices via the receiving means.
The present invention is particularly of use when acquiring data in near-horizontal wells, or horizontal wells, and in such situations the receiving means is preferably located within the vertical part of the wellbore, with the sensing devices adapted to be positioned along the near horizontal or horizontal portion of the wellbore. The use of a receiving means with an electrical connection to surface to carry signals over the vertical section of the wellbore substantially reduces the number of spaced sensing devices required to transmit the signal from downhole to surface over the number required for pure wireless telemetry along the entire length of the wellbore.
Preferably each sensing device has at least one sensor, a transmitter and a receiver. This allows data acquired by one sensor to be transmitted to a receiver on a neighbouring sensing device for transmittal from the neighbouring sensing device to the next neighbour, et seq. Data signals thus may be relayed along the array of sensing devices until the last sensing device in the array transmits all acquired data to the receiving means.
Each sensing device may include a plurality of sensors so that data relating to different parameters, such as temperature, pressure, resistivity, and fluid flow, can be acquired.
Each sensing device may be provided with a processing unit to allow signals received from a sensing device further down the array to be combined with its own signal, so as to produce a combined signal for onward transmission to the next sensing device in the array.
Preferably each sensing device is provided with first and second transceivers, the first transceiver receiving a signal from a first neighbouring sensing device in the array, and the second transceiver transmitting the combined signal to a second neighbouring sensing device in the array.
In some arrangements, the sensing devices may be able to move within the wellbore, and to this end may be in the form of robot-like entities.
In other arrangements, the sensing devices may be permanently fixed in position within the wellbore, typically by incorporating the sensing devices into casing, either internally or externally. With such permanent positioning, the sensing devices require a downhole power supply, which may be provided for example by means of batteries. However the power available downhole will be limited compared to that which would be available with a power connection to surface, and therefore the transmission range of the transceivers associated with the sensing devices will be limited as compared with sensing devices which can be powered from the surface. Boosters may be provided between the sensing devices to ensure that each device is within range of its nearest neighbour or preferably sensing devices may be spaced at intervals along the well such that the distance between each sensing device and its nearest neighbour is not greater than the transmission range.
Thus at the limit of operation, without boosters, each sensing device is spaced apart from its near neighbour by a distance equal to the transmission range. Desirably at least two sensing devices are provided within a distance equal to the transmission range. While this increases the number of sensing devices required downhole, it ensures that a failure of one sensing device does not prevent onward transmission along the array. Thus multiple sensing devices may be provided within the distance equal to the transmission range, and as a preferred example four sensing devices may be provided.
Typically the spacing between the individual sensing devices may range from 3 meters to 100 meters.
According to another aspect of the invention, there is provided a method of acquiring signals down a wellbore, comprising the steps of placing a plurality of spaced apart sensing devices within a lower part of the wellbore so as to form an array, and transmitting data acquired by each sensing device to a next one in the array by wireless telemetry, so as to relay data along the array.
The use of such a relay technique avoids the need for the sensing devices to have a large power output and consequential large power requirement which would be difficult to support where the sensing devices are autonomous and connection to a surface power supply is not desirable or not possible.
The expression xe2x80x9cwireless telemetryxe2x80x9d is intended to include any form of wireless communication, for example using acoustic transmission or electromagnetic waves.
Preferably the method also comprises placing a receiving means attached to an elongate electrical connector within an upper part of the wellbore, and one of the sensing devices of the array transmits data to the receiving means.
These and other features of the invention, preferred embodiments and variants thereof, possible applications and advantages will become appreciated and understood by those skilled in the art from the detailed description and drawings following below.